Method of making temperature responsive actuator

ABSTRACT

There is disclosed a method of making an actuator having a shaft which, responsive to temperature changes, is rotated by bimetallic coiled members. The members are wound into tightly coiled springs so that each produces a high spring torque. One member is wound with the lower-thermal-coefficient-of-expansion layer radially inward and the higher coefficient-of-expansion layer radially outwardly; the other member is with the higher thermal coefficient-of-expansion layer radially inward and the lower coefficient-of-expansion layer radially outward. Each member is clamped into a tight coil configuration. The inner ends of the members are connected to the shaft and the outer ends to a fixed support; the members being positioned about the shaft with their coils wound oppositely so that the spring torques of the members counteract at any temperature but the torques produced by change in temperature act cumulatively. The clamping of the members is then released. The spring torques of the members counteract each other but the torques produced by temperature changes are cumulative.

Shepard METHOD OF MAKING TEMPERATURE RESPONSIVE ACTUATOR [75] Inventor: Basil S. Shepard, Lanham, Md.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: Oct. 25, 1972 [21] Appl. No.: 300,615

Related US. Application Data [62] Division of Ser. No. 167,937, Aug. 2, 1971, Pat. No.

[52] US. Cl. 29/446 [51] Int. Cl 323p 11/02 [58] Field of Search..... 29/446, 173; 73/204, 362.3, 73/3627; 116/129 [56] References Cited UNITED STATES PATENTS 1,468,689 9/1923 Ernst 73/363.7 X

2,000,092 5/1935 Norwood 73/3637 X 2,157,050 5/1939 Bilger et a1 29/446 UX 2,225,941 12/1940 Hall 73/3637 X 2,298,110 10/1942 Crum 73/3623 2,656,592 10/1953 Cataldo 29/446 3,145,759 8/1964 Zelnick et a1... 29/446 X 3,224,075 12/1965 Andersen 29/446 X [451 Mar. 12, 1974 Primary Examiner Charlie T. Moon Attorney, Agent, or FirmC. L. ORourke [5 7] ABSTRACT There is disclosed a method of making an actuator having a shaft which, responsive to temperature changes, is rotated by bimetallic coiled members. The members are wound into tightly coiled springs so that each produces a high spring torque. One member is wound with the lower-thermal-coefficient-ofexpansion layer radially inward and the higher coeffcient-of-expansion layer radially outwardly; the other member is with the higher thermal coefficient-ofexpansion layer radially inward and the lower coefficient-of-expansion layer radially outward. Each member is clamped into a tight coil configuration. The inner ends of the members are connected to the shaft and the outer ends to a fixed support; the members being positioned about the shaft with their coils wound oppositely so that the spring torques of the members counteract at any temperature but the torques produced by change in temperature act cumulatively. The clamping of the members is then released. The spring torques of the members counteract each other but the torques produced by temperature changes are cumulative.

4 Claims, 9 Drawing Figures PATENTEB MARI 2 I974 SHEET 1 BF 3 FIGS FIG.3

METHOD OF MAKING TEMPERATURE RESPONSIVE ACTUATOR CROSS-REFERENCE TO RELATED APPLICATIONS This application is a division of US. Pat. application Ser. No. 167,937 filed Aug. 2, 1971, now Pat. No. 3,720,107, and assigned to Westinghouse Electric Corporation.

BACKGROUND OF THE INVENTION This invention relates to the art of producing actuation of a mechanism responsive to changes in temperature and has particular relationship to such actuation produced by bimetallic means. Typically such actuation is used in automatic chokes for automobiles, in meters and instruments as typified by Trane U.S. Pat. No. 1,288,296, Hall US. Pat. No. 2,225,941, Crum US. Pat. No. 2,298,110 and Bowen US. Pat. No. 2,681,635, and for thermal controllers on space craft.

It is desirable, particularly in space actuation or the like, that the actuations have a high gain; i.e., that for a given change in temperature the actuator produce a high rotational motion or a high torque. It is an object of this invention to provide high-gain actuation.

High-gain actuation is provided by bimetallic coiled members or coils, that is, coils formed of a bimetal having contiguous strips or layers of lower and higher thermal coefficient of expansion. Such a coil or member may be wound with the spacing between successive turns the same or with the spacing between successive turns increasing progressively, typically exponentially from the center or center to the outside. Typically the coils may have a spiral configuration or a helical configuration.

In the use of these coils the outer end of the coil is secured to the stationary structure and the inner end of the coil to a rotatable shaft. The coil, in its free state, then actuates the shaft for any change in temperature.

Actuators in accordance with the teachings of the prior art suffer from the disadvantage that the rotational motion for a giyen change in temperature is low.

A particular shortcoming of the spiral configuration in which the spacing between the windings increases progressively is that the length, and consequently the gain, is limited by the fact that as the coil is wound one layer on top of the next, the diameter even of a tightly wrapped coil soon increases to the point where yielding no longer occurs during the winding operation and no further curvature can be induced in the coil. The length of this spiral coil is further limited by limitations on the outer diameter of the coil. On the other hand, the coil, which has a constant radial spacing between windings, can be made of significantly longer bimetallic strip than the coil with the variable spacing before yielding or packaging problems become significant. A major problem with this desirable constant-spacing configuration, however, is that it requires special and more costly fabrication techniques.

It is an object of this invention to overcome the disadvantages of the prior art and to provide a method of making a temperature-responsive actuator including a bimetallic-coil drive which shall have a high rotational motion or torque for a given change in temperature.

SUMMARY OF THE INVENTION In accordance with this invention an actuator is made which includes a pair of bimetallic members or coils both connected to a common driven shaft or pin at the center and to a support, adjacent the shaft, at the periphery. The members are each wound into tight coiled springs and clamped, one with its strip of lower coefficient of expansion radially outwardly and its strip of higher coefficient of expansion radially inwardly and the other with its strip of lower coefficient of expansion radially inwardly and its strip of higher coefficient of expansion radially outwardly. The so coiled and clamped members are so connected to the shaft that as viewed from any direction along the 'shaft, one member is wound or coiled clockwise and the other counterclockwise.

The inner ends of the members are secured to the shaft while clamped and the outer end to the support and then the clamping is released. The members are properly centered and the spring action causes the turns to separate to a limited extent. The torque exerted by the spring forces of the members counteract each other and the shaft is, at any temperature, held in a fixed position. For any change in temperature both members exert torques in the same direction which causes the shaft to rotate. For given temperature change of the support and the coils, rotation proceeds until the two springs are again in static equilibrium. The rotation theoretically is the same to that which would be obtained if either one of the two coils underwent the same temperature change in the ,free (not prewound) state.

In the interest of reduced cost and ease of production the coils are originally wound in spirals with the spacing between successive turns increasing exponentially, between the center and the periphery. Each coil is then wound tightly with any element of the coil being in contact with the coil immediately above (radially outward) and the coil' immediately below (radially inward). Each wound coil is then secured in a clamping device. When attachment of the coil ends is made as described above and when the clamping devices are removed, the coils assume the configuration of a spiral with the spacing between turns the same. This actuator has the advantage that with the coils of the low-cost variable-turns spacing configuration, the desirability features of constant-turns-spacing coils are achieved.

In the practice of this invention it is desirable to reduce the friction between the turns. Friction may be reduced by use of low-coefficient-of-friction materials for the contacting surfaces of the members or by providing the surfaces with low friction coatings such as polytetrafiuoroethylene or molybdenum disulfide. In spacecraft uses a layer of 0.0003 inch thickness of molybdenum disulfide is provided.

In the practice of this invention it may also be desirable to retain the coiled members or coils from bulging axially along the shaft. This object is achieved by lowfriction restraints suspended from the support.

BRIEF DESORIPTION OF DRAWINGS FIG. 1 is a view in perspective showing an actuator made in the practice of this invention;

FIG. 2 is a view in end elevation of a coiled member used in the practice of this invention in which the spacing between the turns is substantially uniform;

FIG. 3 is a view in side elevation of the member showing in FIG. 2;

FIGS. 4 and 5 are views corresponding to FIGS. 2 and 3 respectively showing a coiled member in which the spacing between the turns increases progressively from the center outwardly, such a coiled member is referred to as spiral in the art;

FIG. 6 is a view in perspective showing the manner in which an actuator as shown in FIG. 1 is made in the practice of this invention;

FIG. 7 is a view in perspective showing apparatus according to the invention in which a louver is controlled by bimetallic coiled means;

FIG. 8 is a diagram presenting an electrical analogue of FIG. 7; and

FIG. 9 is a graph illustrating the operation of FIG. 7.

DETAILED DESCRIPTION OF INVENTION The actuator shown in FIG. 1 includes a shaft 11, which is rotatable in bearings (not shown) and which actuates a mechanism (not shown in FIG. 1) such as the louver 13 shown in FIG. 7. The apparatus shown in FIG. 1 also includes a support 15 and bimetallic coiled members or coils 19 and 21. The peripheral ends 23 and 25 of members 19 and 21 are secured to the support 15 and the inner ends 27 for 19 and 28 (FIG. 6) for 21 are secured in rotating relationship with the shaft 11.

As viewed in the direction of the arrow 29, member 19 is coiled counterclockwise from the center outwardly and member 21 is coiled clockwise.

Each member 19 and 21 is made from strips or layers 31 and 33 and 35 and 37 respectively. Layers 31 and 37, which are radially outward on member 19, and, radially inward on member 21, are of material of higher thermal coefficient of expansion than layers 33 and 35, which are radially inward on member 19, and, radially outward on member 21.

The spacings between the turns of the members 19 and 21 will be essentially uniform, like the spacings S of member 39 shown in FIGS. 2 and 3. They do not in crease progressively from the center outward like the spacings S1 of member 41 shown in FIGS; 4 and 5. Axial bulging of the turns of the members 19 and 21 is suppressed by retaining discs 43, 45 and 46. g

The surfaces of the turns of members 19 and 21 are provided with low-friction coatings 47 (FIG. 1) to suppress frictional forces between the turns. The discs 43,.

45, 46 are also provided with low-friction coatings 48 on their faces which engage the edges of the turns of the members 19 and 21. The support 15 and the members 19 and 21 are maintained at a common temperature.

FIG. 6 shows how the coiled members 19 and 21 are mounted in the actuator shown in FIG. 1. The members 19 and 21 are coiled tightly and held in the tightly coiled state by C clamps or collars 51 and 53 with their outer ends 23 and 25 extending from the openings in the clamps 51 and 53. The clamps 51 and 53 prevent the members 19 and 21 from unwinding. There is a small amount of rotation at the inner ends 27 and 28 where the members first expand to engage the inner surfaces of the clamps. While the coils 19 and 21 are held ends 23 and 25 are secured to the support 15.

There the shaft 1 1 is centered with respect to the coiled members 19 and 21 and the ends 27 and 28 secured to the shaft. The members 19 and 21 are interposed between the discs 43, 45 and 46 and the clamps 51 and p 53 are then removed and the shaft and members 19 and 21 rotate slightly until the torques of the members counterbalance each other. The coiled members 19 and 21 may initially be of the variable-spacing type as shown in FIGS. 4 and 5, with the spacing between turns varying exponentially from the center outward. After mounting as just disclosed the members assume a configuration in which the spacing is substantially uniform as shown in FIGS. 2 and 3.

EXAMPLE An actuator according to the invention was built and tested. All parts engaging the coils 19 and 21 were made of TEFLON (polytetrafluoroethylene) composition and each coil was coated with mold release of TEFLON composition. A system gain of 6/ F. was demonstrated with an error of 1 1 F in 20 F. The overall coil diameter was 1.5 inches as opposed to a diameter of 4.0 inches required for the free spiral coil.

The apparatus shown in FIG. 7 includes the louver 13 which is controlled by coiled members 19 and 21. These members are substantially at the temperature of support 15 which is heated by radiation. Increase in the temperature of the support 15 causes the louver l3 to move in the direction closing the opening 61. A number of such louvers each controlled by a bimetallic coiled actuator meansare provided on a space craft. The following discussion deals with the operation of one louver 13 but its conclusions relate to the operation of a plurality of louvers.

The following analysis develops the structural spacecraft active thermal control system which includes a bimetallic actuator 19 and 21. The most significant result of this analysis is that to minimize the error in angular position at a given temperature, the thermal gain of the bimetallic actuator should be as large as practicable. The bimetallic actuator according to this invention allows the gain value to be increased far beyond that of conventional bimetallic actuators (particularly sprial configuration in which the spacing between the-turns increases progressively) for the same packaging volume. If a small angular position error is required the actuator according to this invention is superior to the conventional prior art actuators. The bimetal members 19-21 sense the temperature T'of the block or support 15 and radiating surface having mass m. If the temperature of the block changes from some reference temperature .T,;, the louver 13 is rotated to some angle 0 i.e., 1

tarded and it would rotate to the angle 0 (FIG. 7) instead of the angle 0 mc,,T Q solar internal Q radiated c; Sin 0 Q, (3', Sin 0 wherein C is a constant of the members 19-21 T is aT/dt, 1 being a time variable Q is the energy received, radiated, or developed as indicated by the subscript.

C is a factor which varies with the position of the space craft and the structure of the louver 13 and the opening 61 which it covers. C., is a constant of the system. Considering the torque generated by the bimetal members as their temperature changes, and its effect on the louver 13.

E torques K (0 0) f= J5 where K spring constant of bimetal members.

f= friction torque J moment of inertia of louver blade Therefore, KO J5 +f+ K0 Note thatfmay be either viscous or coulomb or some combination of both. It can be assumed in the present case thatfis coulomb. A maximum value off can be set based on the allowable static errors that the system can tolerate. When the bimetal members change temperature, the louver 13 does not move until enough torque is developed to overcome the static friction f Let ()2 0 0- where 6e is the angle error produced by'static friction.

f Kfi f: i e

e =fs/K 1 where T is the temperature error. If the temperature i.e., AB f fd d0 (PE), (PE) (PE) (l/2)K0 w where 0e initial static angular error =f /1T 0 w static error immediately after the louver breaks initial friction, moves and stops.

51f; EH35]??? Ill K K 0 w =(1/K) [Zf f,,]; (f /K) (trivial solution,

e:fs fd) FIG. 9 is a plot of 0 against a, 0 being plotted vertically and a horizontally. I

Note that |0u| |0 only ifa l, i.e., the dynamic friction is greater than the static friction which is physically unrealizable. The usual case is for a S 1. Therefore, the maximum temperature error is the static temperature error:

for equation 1 c (at) C C086 50 so, Sin mac C'4 Cos 086 for equation 2 where 5C' represents variations in the solar input be-' cause of orbital and satellite altitude variations.

use, J 66 8f+ K for equation 5 where 8f 5f (0) Rewriting (9) (ST) c,a0 0,50 80, +80,

where (Sin 68C' is simply restated as 8 Q,,, the variation in solar load.

The control diagram for equations (8), (l0) and (1 l) is shown in FIG. 8 where it may be noted that if C is greater than C.,, then the feedback is positive and the louver system is unstable, i.e., a solar input heats the radiating surface, causing the louver to open more, thereby increasing the absorbed solar energy causing further opening, etc.

Stability in terms of the Routh-Hurwitz criterion requires that:

(l/wn O where the characteristic equation is 1+ GH S[(l/um (2T/mn) S 11+ C C /C and T is the equivalent damping factor.

In other words, it is required that the friction be nonzero, which in practice is readily obtained, and that the friction be greater than some value based on the gain factors associated with the spring constant (C of the bimetal members, the radiator (C and the thermal capacity associated tha mass attached to each louver (C An equivalent value of T for a system with cou lomb friction may be obtained as follows. It is assumed that cyclic motion occurs and when it does, each system, i.e., the viscous arid coulomb systems, dissipates the same energy.

For the viscous system:

AE/cyc'= (1) H119 qb (FdB/dt) dt d) FBdt d) C0 dt where C ZTwnJ Assuming cyclic motion, let 9 A sin wt E/cyc CA (b m Sin wt dt CA w (1),, 211'Sin (wt) d (out) CA /zX (1/4) Sin X] 2'n' For a coulomb friction system, also assuming cyclic motion,

' AE/cyc (11 Fd d0 4f A Comparing systems and equating energy loss per cycle,

For maximum specified static error it is required that As an example for demonstrating feasibility, one can evaluate the stability requirements taking values which are applicable in a practical situation system. Taking a conservative approach, let C C (i.., C =O), and let C KT instead of C f, KT,

It is required that C4 ')(fd/ z/ 1) f1( A)(C /fs) KTe (4/1r)(f fs)(wn TeC /mA) For (for w /24 louvers S/louver). To be extra conservative let w out instead of being equal to the orbital rate, and select A to be as high as possible, namely A 1r/4 radians.

Therefore,

C (4/1'r)(1/2) (24) (3) (1.0) /1r/4 C 58.3 BTU/sec rad X (3,600 sec/hr./3.4 BTU/watt C 61,800 watts/rad This is easily satisfied. It is also desirable to maintain C C so that the louvers do not open when exposed to solar inputs. This usually requires shades if the orbital geometry permits solar energy to enter the louver system. The remaining condition is that This condition may be met by minimizing f, by reducing static friction or by maximizing K and/or Cl fir l e The actuator made with practice of this invention has the following principal advantages:

1. For the same functional and packaging requirements, this actuator can be manufactured more readily and at a lower cost than the prior-art actuator with a coil with equally spaced turns because of the special machinery required to wind such a coil.

2. For a given length, shaft diameter, overall width and maximum allowable outer diameter, the actuator made in the practice of this invention produces a greater torque than the prior art. This is because a thicker material may be used. This is especially important for minimizing static errors in a louver system.

3. Conversely, for a given shaft diameter, material thickness and maximum outer diameter, the length of the bimetallic material and hence the gain of the actuator made in the practice of this invention can be increased far beyond that of the prior-art actuators.

4. For a given shaft diameter, material thickness, and desired gain, the actuator made in the practice of this invention can be packaged in significantly less volume than the prior art actuators.

5. The system including the actuator made in the practice of this invention is a stiffer system in both the axial and radial directions and thus is advantageous in vibration and shock environments.

6. The fact that all turns of the coil configuration of the actuator made in the practice of this invention are in closer proximity to one another than the turns of the prior art actuator helps to assure a more uniform temperature along the length of the coil and reduces the thermal time constant by improving the view factor between coil turns.

While specific practice of this invention has been disclosed herein many modifications thereof are feasible.

This invention is not to be restricted except insofar as in necessitated by the spirit of the prior art. I

I claim:

1. The method of producing a high gain temperature responsive actuator including a shaft, supporting means contiguous to said shaft, a first bimetallic member and a second bimetallic member, each member having a first strip of higher thermal coefficient of expansion and a second strip of lower thermal coefficient of expansion, the said method comprising winding said first member into a tight coiled spring having the first strip radially outward and the second strip radially inward and clamping said member in the tight configuration, winding said second member in a tight coiled spring having the second strip radially outward and the first strip radially inward, securing the inner ends of said members to said shaft and the outer ends of said members to said support with said members positioned so that said springs are wound oppositely, and releasing the clamping of said members, whereby at any temperature the spring torques of said members counteract each other and responsive to a change in temperature, the resulting torques produced by said members act in the same direction to rotate said shaft.

2. The method of claim 1 wherein each member is wound with the spacing between successive turns increasing progressively from the center outward and after the clamping on said members are released each said member answers a configuration in which the spacings between the turns are substantially the same.

members are released. 

1. The method of producing a high gain temperature responsive actuator including a shaft, supporting means contiguous to said shaft, a first bimetallic member and a second bimetallic member, each member having a first strip of higher thermal coefficient of expansion and a second strip of lower thermal coefficient of expansion, the said method comprising winding said first member into a tight coiled spring having the first strip radially outward and the second strip radially inward and clamping said member in the tight configuration, winding said second member in a tight coiled spring having the second strip radially outward and the first strip radially inward, securing the inner ends of said members to said shaft and the outer ends of said members to said support with said members positioned so that said springs are wound oppositely, and releasing the clamping of said members, whereby at any temperature the spring torques of said members counteract each other and responsive to a chAnge in temperature, the resulting torques produced by said members act in the same direction to rotate said shaft.
 2. The method of claim 1 wherein each member is wound with the spacing between successive turns increasing progressively from the center outward and after the clamping on said members are released each said member answers a configuration in which the spacings between the turns are substantially the same.
 3. The method of claim 1 which includes the step of providing the strips which form the turns of each coiled member with low-friction surfaces so that the friction between contiguous turns, that may be in contact, of each of the coiled members is minimized.
 4. The method of claim 1 which includes the step of suppressing the axial bulging of the coils when the members are released. 